Paired lines are a conventional means of carrying telecommunications transmissions. A paired line is made up of two balanced conductors individually insulated and twisted together. Paired lines are typically bunched together in a cable termed a paired cable, which contains up to one hundred or more paired lines, wherein each paired line is capable of independently carrying telecommunications signals. Paired lines are generally effective telecommunications carriers. However, it is not unusual for noise to occur in paired lines which is extremely disruptive to the clarity of the transmitted signal.
When noise is reported in a paired telecommunications line, correction of the condition requires confirming the presence of the noise in the line by measuring its level and then isolating and locating the noise source for purposes of eliminating it. Presently known methods for performing these tasks are to simply tap into the line and either listen for the noise on a handset or measure the noise level with a passive noise measurement set. However, both of these methods draw dc loop current from the paired line in order to operate. The flow of dc loop current from the paired line into the detection device tends to seal any noise-causing faults in the line, rendering the faults essentially undetectable. Consequently, the noise will continue to be apparent to a line user, but the noise source will elude detection and repair.
There are a wide range of noise sources for which detection is desirable since virtually any condition which can cause an imbalance between two conductors of a paired line can result in noise. Among the causes are series resistance faults, shunt resistance faults, cross faults, shunt capacitance faults, unbalanced series inductance, and power influence. Series resistance faults occur when there is an open in a line, often resulting from a corroded joint. Shunt resistive faults occur when another body grounds a paired line. Cross faults occur when there is communication between adjacent paired lines in a cable. Shunt capacitance faults occur when one conductor of a pair is slightly longer than the other conductor, and the longer conductor possesses a higher capacitance to ground than the shorter conductor. Unbalanced series inductance occurs when only one half of a load coil is connected to a paired line at some point along the length of the line. Power influence is induced voltage from an ac power source adjacent the paired line. Unlike the above-recited causes of imbalance, power influence imbalance can occur even when the paired line is free of faults and appears balanced in the absence of the power influence.
Power influence, which as noted above is induced voltage from line to ground, most commonly occurs when the paired line is near a power line. In the United States, the power line frequency is typically 60 Hz, but power influence can likewise result from other power line frequencies, including 50 Hz, as typically found in many other parts of the world. Power influence can create unique problems for noise detection when it occurs in conjunction with a fault. For example, a series resistance fault may only produce a high level of noise when accompanied by a high power influence. Therefore, a noise caused by the fault may be observed by a user at a time of high power demand on a nearby power line, but when a repairman is dispatched to the site, the power demand and correspondingly the power influence may have diminished so that the noise resulting from the fault alone is no longer detectable by conventional detection devices. Accordingly, such a fault is very difficult to locate and repair.
Another detection problem which occurs when power influence is present in conjunction with a fault results from the fact that power influence signals often do not create large longitudinal current flow. Such flow is necessary to detect series resistance faults because longitudinal current flow through a series resistance fault produces a voltage imbalance in the paired line which can be measured metallically. However, because conventional passive detection devices lack the ability to independently generate longitudinal current flow, they accordingly may fail to detect such faults where power influence is relied upon to generate longitudinal current flow.
As such, it is apparent that a need exists for a reliable noise measurement device which provides more certainty of noise detection on a paired line than do existing devices. Specifically, a noise measurement device is needed which can detect noise on a paired line from a wide range of sources and in particular from series resistance faults and power influence even at times of low power demand. A noise measurement device is needed which does not modify or obscure the noise signal on the paired line when the device is introduced into a loop containing the noisy line. A noise measurement device is needed which can independently generate longitudinal current flow when necessary in the paired line being tested. Further, a noise measurement device is needed which can effectively locate noise sources on the paired line when it is working, i.e., wet, as well as when it is non-working, i.e., dry.